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Register a new userThe SMC X-ray binary SXP4.78 : a new Type II outburst and the identification and study of the optical counterpart
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SXP4.78 was originally discovered in 2000 as a pulsar in the Small Magellanic Cloud (SMC) by the Rossi X-ray Timing Explorer (RXTE) but it was not spatially located at that time. A new detection in 2018 with the Neil Gehrels Swift Observatory during a Type II outburst permitted its position to be accurately located and its optical counterpart identified. We report X-ray and optical monitoring covering epochs before and during the outburst. Using photometric data we show the long-term variability of the Be disc where we present flux and colour changes associated with the disc growth and decay over a period of ~6000 days. We show evidence of disc growth during the recent outburst through an increase in the H-alpha equivalent width and photometric flux. Period analysis was performed using both optical photometric and spectroscopic data, but with no significant detection of an orbital period. A modest periodic signature of 2.65 days was detected from the OGLE I band data, however, but we attribute that to the non-radial pulsations (NRPs) of the Be star. We also obtained a blue spectrum from the Southern African Large Telescope (SALT) which permits us to classify the spectral type as B0.5 IV-V.
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Optical and X-ray observations are presented here of a newly reported X-ray transient system in the Small Magellanic Cloud - SXP7.92. A detailed analysis of the X-ray data reveal a coherent period of 7.9s. A search through earlier X-ray observations of the SMC reveal a previously unknown earlier detection of this system. Follow-up X-ray observations identified a new transient source within the error circle of the previous observations. An optical counterpart, AzV285, is proposed which reveals clear evidence for a 36.8d binary period.
In this paper we report on the optical and X-ray behaviour of the Be X-ray binary, SXP 91.1, during a recent type I outburst. We monitored the outburst using the Neil Gehrels Swift Observatory. These data were supported by optical data from the Southern African Large Telescope (SALT) and the Optical Gravitational Lensing Experiment (OGLE) to show the circumstellar disc activity. Matter from this disc accretes onto the neutron star, giving rise to the X-ray outburst as seen in the synchronous evolution of the optical and X-ray lightcurves. Using data taken with OGLE we show that the circumstellar disc has exhibited stable behaviour over two decades. A positive correlation is seen between the colour and magnitude from the OGLE and MACHO observations, which indicates that the disc is orientated at relatively low inclination angles. From the OGLE and Swift data, we demonstrate that the system has shown relative phase offsets that have persisted for many years. The spin period derivative is seen to be at maximum spin-up at phases when the mass accretion rate is at maximum. We show that the neutron star in SXP 91.1 is an unusual member of its class in that it has had a consistent spin period derivative over many years, with the average spin-up rate being one of the highest for known SMC pulsars. The most recent measurements of the spin-up rate reveal higher values than the global trend, which is attributed to the recent mass accretion event leading to the current outburst.
On 2016 July 30 (MJD 57599), observations of the Small Magellanic Cloud by Swift/XRT found an increase in X-ray counts coming from a position consistent with the Be/X-ray binary pulsar SMC X-3. Follow-up observations on 2016 August 3 (MJD 57603) and 2016 August 10 (MJD 57610) revealed a rapidly increasing count rate and confirmed the onset of a new X-ray outburst from the system. Further monitoring by Swift began to uncover the enormity of the outburst, which peaked at 1.2 x 10^39 erg/s on 2016 August 25 (MJD 57625). The system then began a gradual decline in flux that was still continuing over 5 months after the initial detection. We explore the X-ray and optical behaviour of SMC X-3 between 2016 July 30 and 2016 December 18 during this super-Eddington outburst. We apply a binary model to the spin-period evolution that takes into account the complex accretion changes over the outburst, to solve for the orbital parameters. Our results show SMC X-3 to be a system with a moderately low eccentricity amongst the Be/X-ray binary systems and to have a dynamically determined orbital period statistically consistent with the prominent period measured in the OGLE optical light curve. Our optical and X-ray derived ephemerides show that the peak in optical flux occurs roughly 6 days after periastron. The measured increase in I-band flux from the counterpart during the outburst is reflected in the measured equivalent width of the H-alpha line emission, though the H-alpha emission itself seems variable on sub-day time-scales, possibly due to the NS interacting with an inhomogeneous disc.
We report the Chandra/HRC-S and Swift/XRT observations for the 2015 outburst of the high-mass X-ray binary (HMXB) pulsar in the Small Magellanic Cloud, SMC X-2. While previous studies suggested that either an O star or a Be star in the field is the high-mass companion of SMC X-2, our Chandra/HRC-S image unambiguously confirms the O-type star as the true optical counterpart. Using the Swift/XRT observations, we extracted accurate orbital parameters of the pulsar binary through a time of arrivals (TOAs) analysis. In addition, there were two X-ray dips near the inferior conjunction, which are possibly caused by eclipses or an ionized high-density shadow wind near the companions surface. Finally, we propose that an outflow driven by the radiation pressure from day ~10 played an important role in the X-ray/optical evolution of the outburst.
The Small Magellanic Cloud (SMC) Be/X-ray binary pulsar SXP6.85 = XTE J0103-728 underwent a large Type II outburst beginning on 2008 August 10. The source was consistently seen for the following 20 weeks (MJD = 54688 - 54830). We present X-ray timing and spectroscopic analysis of the source as part of our ongoing Rossi X-ray Timing Explorer (RXTE) monitoring campaign and INTEGRAL key programme monitoring the SMC and 47 Tuc. A comparison with the Optical Gravitational Lensing Experiment (OGLE) III light curve of the Be counterpart shows the X-ray outbursts from this source coincide with times of optical maximum. We attribute this to the circumstellar disk increasing in size, causing mass accretion onto the neutron star. Ground based IR photometry and H-alpha spectroscopy obtained during the outburst are used as a measure of the size of the circumstellar disk and lend support to this picture. In addition, folded RXTE light curves seem to indicate complex changes in the geometry of the accretion regions on the surface of the neutron star, which may be indicative of an inhomogeneous density distribution in the circumstellar material causing a variable accretion rate onto the neutron star. Finally, the assumed inclination of the system and H-alpha equivalent width measurements are used to make a simplistic estimate of the size of the circumstellar disk.
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